The present invention relates to an apparatus and a method for detecting the absolute position of a rotary body such as a steering wheel.
An electric power steering apparatus, in which steering is assisted by an electric motor, has recently been proposed to reduce the steering force of a steering wheel of a vehicle. In controlling the electric power steering apparatus, it is required to determine the absolute position (absolute steering angle) of the steering wheel. The steering wheel is rotated within the finite rotation number, which is more than or equal to one rotation. The absolute position of the steering wheel is the angle by which the steering wheel is rotated from the neutral position. The neutral position is the position of the steering wheel at which the vehicle travels in a straight line.
A steering sensor is used as a sensor for detecting the absolute position of a steering wheel. The steering sensor includes a slit plate and three sets of photo interrupters. The slit plate has several slits and is arranged on the steering wheel to rotate integrally with the steering wheel. The photo interrupters are secured to a steering column. Each set of photo interrupter includes a light emitting diode and a photodetector, which face each other. The slit plate, which is inserted between the light emitting diode and the photodetector, is rotated so that the beam between the light emitting diode and the photodetector is selectively blocked and unblocked. Accordingly, the photodetector sends out signals.
Two sets of photo interrupters detect the number of rotation and the rotational direction of the slit plate. The remaining one set of photo interrupters detects the neutral position (reference position) of the steering wheel in one rotation. Output signals are, for example, sent to a control unit such as an electronic control unit (ECU) mounted on a vehicle.
The steering wheel can generally be rotated more than one rotation (360 degrees). The steering wheel is, for example, rotated two rotations (720 degrees) to the left and two rotations (720 degrees) to the right starting from the neutral position. Thus, even when a signal representing the neutral position is sent from one set of photo interrupter used for detecting the reference position, it is not clear how many times the steering wheel has been rotated. Therefore, it is impossible to detect an accurate absolute position of the steering wheel only by using the detection signals from the steering sensor.
Also, the accurate absolute position of the steering wheel cannot be detected immediately after an ignition switch is turned on. When a vehicle starts traveling and one set of photo interrupters detects the neutral position, the control unit detects the number of rotation and the rotational direction of the steering wheel from the neutral position based on the output signal from the first set of photo interrupter and the output signals from the other two sets of photo interrupters. However, the neutral position detected immediately after the ignition switch is turned on does not show how many times the steering wheel has been rotated. Thus, the accurate absolute position is detected based on output signals from the three sets of photo interrupters, and the detection signals from the vehicle speed sensor and the yaw rate sensor after the vehicle travels further. The control unit of the vehicle, which uses the absolute position as a parameter, is operated based on the detected absolute position.
It is desired to reduce the cost for detecting the absolute position of the steering wheel. However, the steering sensor that has the above mentioned complicated structure must be used for detecting the absolute position of the steering wheel. The above described prior art steering sensor requires the vehicle to travel a certain distance from when the ignition switch is turned on until the sensor detects the accurate absolute position. Thus, the steering sensor cannot detect the absolute position in earlier stage.
The electric motor located in the electric power steering apparatus includes a motor resolver, which detects the rotational angle of the motor. The electric power steering apparatus includes a torque sensor, which has a resolver for detecting the steering torque of the steering wheel. The motor resolver and the torque sensor send resolver signals, or rotational angle signals. Thus, it is considered that detection of the absolute position using these signals will reduce the cost. However, even when the absolute position is detected using the resolver signals from the motor resolver, it is difficult to specify the absolute position of the steering wheel since the position of the rotor is specified by the electrical degree within one cycle only.
The resolver, which is located in the torque sensor, detects the torsion of a torsion bar. The torsion bar is incorporated in a pinion shaft, which is coupled to the steering wheel. However, since several signals are sent out in one rotation of the steering wheel, it is not clear what signal number from the neutral position of the steering wheel is detected.
Therefore, the absolute position of the steering wheel cannot be detected using the signals sent from each resolver independently.